1. Field of the Invention
The invention relates generally to transparent, superhydrophobic thin film coatings.
2. Description of the Related Art
There are many practical applications for superhydrophobic thin film coatings. They can be used on car windshields, sky-scraper windows, and binocular lenses to both improve visibility and protect from water corrosion. They can also be used to weatherproof solar panels and overhead power lines. In the past, such coatings have been tested on either metallic substrates or glass surfaces that have been roughened at the nano-scale. These tests have generally resulted in what are considered superhydrophobic surfaces. The criteria for a hydrophobic or superhydrophobic surface are based on contact angle. This is the angle that the contour of a water droplet makes with the flat substrate. A hydrophobic surface must have a contact angle of 90° or greater, while a superhydrophobic surface must have a contact angle of at least 150°.
The hydrophobicity of solutions containing polydimethylsiloxane (PDMS), such as the car-care product RAIN-X® is known. PDMS, once cross-linked and polymerized, forms a hydrophobic surface with an average contact angle of 110°.
Coating a silicate material via reactive-silane technology has also been used to produce hydrophobic surfaces. Typically, chlorosilane derivatives of long-chain fatty acids, either as the parent hydrocarbon or as a perfloro or polyfluoro derivative, are reacted with surface hydroxyl groups to produce a covalent bond between the hydrophobizing agent and the surface. In the case of the chloro-based derivatives, hydrochloric acid, a toxic, corrosive gas, is produced. Another limitation of this technology includes the high expense of the reactive-silanes.
A need remains, however, to use low-cost, environmentally friendly materials to develop an optically transparent, hydrophobic coating with properties comparable to existing hypdrophobic solutions, such as RAIN-X®. Water droplets on RAIN-X® coated glass will bead up rather than spread out over the glass, increasing visibility during rainfall.
Information relevant to attempts to address these problems can be found in the related art. However, the related art suffers from one or more disadvantages, which are not admitted to have been known in the art by inclusion in this section.
Fatty acids are both inexpensive and have a long, hydrophobic carbon chain ending with a hydrophilic carboxylic acid. In theory, the carboxylic acid will bond to the hydroxyl groups in borosilicate glass, leaving the water repellant carbon chains standing on end. Previous studies used an aluminum alloy surface etched with HNO3 as the substrate, then coated with stearic acid/ethanol solution to lower the surface energy. See Wang et al. “Fabrication of Superhydrophobic Surfaces on Engineering Material Surfaces with Stearic acid.” Applied Surface Science 254 (2008) 2009-2012.
Others have used a similar etching technique, but finished by coating with a lower molecular weight fatty acid such as lauric acid dissolved in ethanol. See: Degang Xie, Wen Li. “A novel simple approach to preparation of superhydrophobic surfaces of aluminum alloys.” Applied Surface Science 258 (2011): 1004-1007. The etching process roughens the substrate, causing air pockets to form, which when coated with a fatty acid, results in a superhydrophobic surface.
None of the information included in this section is admitted to be prior art with respect to the present invention by inclusion in this section.